This type of steel provides:
very high mechanical strength, but at the same time high toughness and ductility, in other words low sensitivity to brittle failure; this very high strength subsists under hot conditions up to temperatures of the order of 400° C.;
good fatigue properties, which notably implies the absence of harmful inclusions such as nitrides and oxides; this characteristic has to be obtained by a suitable composition and careful conditions for elaborating the liquid metal.
Further, it is carburizable and nitridable, so as to be able to harden its surface in order to give it good resistance to abrasion and lubricated friction.
Conceivable applications of this steel concern all fields of mechanics where structural or transmission parts are required which have to combine very high loads, under dynamic stresses and in the presence of induced or surrounding heating. In a non-exhaustive way, mention will be made of transmission shafts, gearbox shafts, bearing axles, . . . .
The requirement of excellent hot mechanical strength in certain applications prevents the use of carbon steels or so-called “weakly alloyed” steels, the strength of which starts to degrade from 200° C. Further, the toughness of these steels is generally no longer satisfactory when they are treated for mechanical strength levels of more than 2,000 MPa, and, generally, their “true” yield strength is much smaller than their maximum strength measured in the tensile test: the yield strength is therefore a dimensioning criterion which becomes a penalty in this case. Maraging steels may then be used, for which the yield strength limit is notably closer to their maximum tensile strength value, which have satisfactory strength up to 350-400° C., and which further provide good toughness for very high mechanical strength levels. But these maraging steels quite systematically contain high nickel, cobalt and molybdenum contents, all elements which are costly and subject to notable variations of their quotation on the commodity market. They also contain titanium, used for its strong contribution to secondary hardening, but which is mainly involved in the lowering of the fatigue strength of maraging steels due to the nitride TiN, the formation of which is quasi impossible to avoid during the smelting of steels even containing only a few tenths of percents.
In U.S. Pat. No. 5,393,488, a steel composition with secondary hardening was proposed without addition of titanium, aiming at improving hot strength and especially improving fatigue properties, ductility and toughness. This composition has the drawback of requiring a high Co content (8 to 16%), which makes the steel very expensive (N.B.: in the present text, all the contents of the different elements are expressed in weight %).
In document WO-A-2006/114499, a composition of hardened martinsitic steel was proposed as well as an optimized series of heat treatments adapted to this composition which, relatively to the prior art represented by U.S. Pat. No. 5,393,388, have the advantage of only requiring a more reduced cobalt content i.e. between 5 and 7%. By adjusting the contents of the other elements and the parameters of the heat treatments, accordingly, it was possible to obtain parts providing a set of very satisfactory mechanical properties, notably for aeronautical applications. These notably are a cold tensile strength comprised between 2,200 MPa and 2,350 MPa, ductility and resilience at least equal to those of the best high-strength steels, and under hot conditions (400° C.), a tensile strength of the order of 1,800 MPa, as well as optimum fatigue properties.
This steel is said to be “with duplex hardening” since its hardening is obtained by simultaneous hardening precipitation of intermetallic compounds and of carbides of the M2C type.
However, this steel always contains relatively large amounts of cobalt. As this element is in any case expensive and its price is likely to be subject to significant fluctuations on the commodity market, it would be important to find means for further reducing its presence very substantially, notably in materials intended for more ordinary mechanical applications than aeronautical applications.
With the steels as proposed in WO-A-20061114499 and U.S. Pat. No. 5,393,488, it is possible to obtain good resilience but for certain applications the latter may prove to be insufficient.
For the same applications, obtaining a very high tensile strength (Rm) is also required.